1. Field of Invention
The present invention pertains to logging tools for use in a wellbore, particularly logging tools having housings made of soft base material relative to the hardness of the wellbore wall or casing disposed in the wellbore.
2. Related Art
Logging tools are commonly used, in oil and gas exploration, for example, to ascertain or infer properties of the subsurface formations encountered by a wellbore. Logging tools may be used while drilling the wellbore, or may be run into the wellbore after drilling, for example, on a wireline. Various types of logging tools may be run, depending on the measurement type. Such measurement types may include, but are not limited to, resistivity, nuclear magnetic resonance (NMR), gamma ray, spontaneous potential, and dielectric constant.
Generally, the bulk of a logging tool is made of very strong material, such as steel. However, often a portion of the tool contains sensors that must communicate in some way with the surrounding environment. For example, resistivity sensors require electromagnetic signals to pass into and from the formation so that information characterizing the formation properties can be obtained. For the signals to pass into or be received from the formation, the sensors are preferably mounted on an electromagnetically transparent medium. Such transparent media may comprise composite, non-metallic materials. A disadvantage to the composite, non-metallic material is its relative softness compared to the formation or casing. In many cases, that relative softness allows wear and tear of the sleeve to occur at an unacceptable high rate. For example, a NMR tool has powerful magnets that are strongly attracted to the steel casing through which the tool must pass before reaching the uncased portion of the wellbore. The magnetic force causes the tool to be dragged against the casing, causing scraping and wear.